A sample which is periodically stimulated by electricity, laser radiation, or other means returns to its original state. It has been required in various applications to know the reaction during this returning process. One application resides in the case in which the properties of a liquid crystal are evaluated. For the above-described measurement, time-resolved Fourier transform spectroscopy using a Fourier transform infrared spectrometer is available.
The present applicant has already proposed a time-resolved spectroscopy utilizing Fourier transformation and an instrument used in this spectroscopy. In particular, a stimulus-generating means repeatedly gives a stimulus to a sample at intervals longer than the duration of response of the sample. Radiation emerging from the sample is detected by a detector through a rapid scan interferometer. An interferogram is obtained from the output from the detector after a given delay with respect to each stimulus. The interferogram is Fourier-transformed to derive a spectrum. In this way, the reaction of the sample which responds equally to every stimulus is investigated. The output signal from the detector is gated onto a low-pass filter after a given delay with respect to the application of each stimulus to obtain the envelope of the signal.
The present applicant has also proposed other time-resolved spectroscopy and instrument. Specifically, a pulsed light source is used as the light source. Light is emitted from this light source at the same intervals as the intervals at which a stimulus is given after a given delay with respect to each stimulus. The output from the detector is passed through a low-pass filter to obtain the envelope of the signal. In this manner, an interferogram representing the state of the sample assumed after a given delay with respect to each stimulus is obtained.
In these proposed methods and instruments, each stimulus can be given asynchronously with the reference signal produced for the interferometer. Therefore, the limitations imposed on the stimulus can be reduced greatly. Also, where a fast reaction takes place, the stimulation frequency can be increased, so that the efficiency of measurement can be enhanced. The output signal from the detector is fed to a gate circuit. Similar measurements are performed while controlling either the delay time of the gate circuit or the delay time of the pulsed light source. As a result, a series of spectra can be obtained according to successively varied delay times.
The present applicant has also proposed an instrument consisting of plural measuring systems for one sample, the measuring systems having a common optical system beginning with a light source and ending with a detector. The measuring systems have their respective delay times and are arranged in parallel. Where the sample under investigation responds equally to every stimulus repeatedly applied, the various states of the reacting sample which correspond to different delay times are measured simultaneously.
Furthermore, the present applicant has proposed a further Fourier transform spectroscopy using a pulsed light source and an instrument used in this spectroscopy. Specifically, an interferogram is taken, using an interferometer. The interferogram is Fourier-transformed to obtain a spectrum of a sample, for analyzing it. The pulsed light source emits light at intervals shorter than the sampling interval. The interferogram consisting of low-frequency components is obtained from the output from the detector, sampled, and Fourier-transformed. As a result, a spectrum of the sample which is helpful in analyzing it is derived. These proposed methods are described in U.S. patent application Ser. Nos. 07/577,636 (now U.S. Pat. No. 5,021,661) and No. 07/676,576.
These methods proposed by the present applicant assume that the intervals at which the stimulus is given or the emission interval , of the pulsed light source is shorter than the sampling interval of the interferogram, i.e., the sampling theorem states that the emission interval of the pulsed light source is shorter than the reciprocal of the square of the maximum frequency f.sub.max of the interferogram signal. This reciprocal is equal to or greater than the sampling interval. That is, the proposed methods assume that .tau.&lt;1/2f.sub.max or f.sub.max &lt;1/2.tau..